The invention concerns a method and device to control the drive of a conveyor device in the form of an escalator or moving sidewalk, switchable between load operation and no-load operation. The conveyor device comprises a line voltage connection that delivers an essentially constant line frequency, an electric drive motor, especially in the form of an induction motor or synchronous motor, and a frequency changer.
A typical conveyor device for personal conveyance in the form of an escalator or moving sidewalk includes a number of closely adjacent steps that are moved by means of a drive motor in the form of an endless belt in the desired direction of conveyance.
To reduce the power consumption and wear of such conveyor devices, a switch has been made to place such conveyor devices in transport movement only when needed, otherwise bringing them to a stop. For this purpose, a transport requirement signal device is provided, for example in the form of a foot mat, light barrier or manually operated switch arranged in front of the conveyor device in the direction of conveyance by means of which the presence of a requirement for transport can be established. If the transport requirement is present, for example, because a person has walked on the foot mat, the conveyor device is placed in forward movement for a predetermined time and switched off again when no further transport requirement has been established within a predetermined time.
To avoid peak loads during frequent engagement and disengagement of the conveyor device, it is known from WO 98/18711 not to switch the drive motor on and off abruptly but to have the speed of the drive motor rise or fall ramp-like during the switching processes. Induction motors are mostly used for such conveyor devices. Since the speed of an induction motor depends on the frequency of the ac power mains, which means constant speed of the induction motor when directly fed from an ac system with constant line frequency, a controllable frequency changer is used with which the line frequency fed to it can be controllably converted to an output frequency different from the line frequency.
The cost for a frequency changer that supplies the drive motor of an escalator or moving sidewalk indeed during load operation would be high, since the costs of the frequency changer rise enormously with the output power that the frequency changer must be able to deliver.
In order to keep the acquisition and operating costs low, WO 98/18711 proposes that the conveyor device only be driven with full transport speed in load operation, and only with reduced no-load speed in standby operation or no-load operation, during which no transport requirement exists, and that the drive motor only be fed from the frequency changer during no-load operation and switching processes, but directly from the line voltage source during load operation. This creates the possibility of laying out the frequency changer much smaller in terms of maximum power, which leads to a significant cost saving relative to a frequency changer whose maximum power is adapted to load operation of the conveyor belt. The conveyor device known from WO 98/18711 then converts to no-load operation, if no additional transport requirement is reported after executing a transport order, and is only shut down when no additional transport requirement is reported for a predetermined time after switching into no-load operation.
With the mentioned measures, a significant reduction of load peaks and abrupt speed changes of conveyor devices is achieved. However, unduly high transient currents can still occur during changing between line feed and frequency changer feed of the drive motor, because of deviations between the line frequency and the output frequency of the frequency changer and their phase positions at the time of switching between line feed and frequency changer feed of the drive motor and because of the actual voltage of the drive motor, which can lead to overloading of the frequency changer and to jerky movement changes of the conveyor device.
Such phenomena are overcome with a method disclosed in the republished previous German Patent Application 199 60 491.6 of the applicant and in which the line voltage and the frequency changer output voltage are compared to each other with respect to frequency and phase position and the frequency changer is controlled at an output frequency that has a predetermined frequency spacing from the line frequency. If a requirement for switching of the conveyor device from load operation to no-load operation or vice-versa is reported by means of a transport signal device, a switching control signal that triggers switching of the drive motor between frequency changer feed and line feed is generated at the time after signaling of this operational switching requirement, at which the output frequency of the frequency changer has both the predetermined frequency spacing relative to the line frequency and also a predetermined phase spacing between the frequency changer output frequency and the line frequency. By not issuing the switching control signal at the time at which the output frequency of the frequency changer agrees with the line frequency both in terms of frequency and phase, but in advance of the time at which the output frequency of the frequency changer has the predetermined frequency spacing relative to the line frequency and has also reached the predetermined phase spacing between the frequency changer output frequency and the line frequency, it is allowed that the switching devices used for switching between no-load operation and load operation, usually contactors, do not operate free of delay, on the one hand, and that, on the other hand, a currentless period is required between dropout of one contactor and pickup of the other contactor in order to avoid shorting of the line voltage through the frequency changer. There is a certain inherent delayed reaction between release of a switching control signal and dropout of the previously conducting contactor and finally pickup of the other contactor, which depends on the specific components of the specific conveyor device and is allowed for by the mentioned frequency spacing and the mentioned phase spacing.
The method described in German Patent Application 199 60 491.6 has proven itself well. However, there are cases in which one would want to get by with lower control costs and this should be achieved with the present invention.
The present invention concerns a method for control of the drive of a conveyor device, especially in the form of an escalator or moving sidewalk, switchable between load operation and no-load operation and having a drive motor and a frequency changer controllable at least with respect to the frequency and phase position of its output voltage, in which the drive motor in load operation is fed with a line voltage with an essentially constant line frequency and in no-load operation with the frequency changer output voltage, the phase difference between the phase position of the line voltage and the phase position of the frequency changer output voltage is determined, the phase position of the frequency changer output voltage is corrected according to the determined phase difference and is therefore brought essentially into agreement with the phase position of the line voltage, and switching is initiated as soon as this agreement is reached.
On the other hand, the invention concerns an electrical control device to control the drive of a conveyor device, especially in the form of an escalator or moving sidewalk, switchable between load operation and no-load operation and having a line voltage connection to supply a line voltage with essentially constant line frequency and a drive motor, having a frequency changer controllable at least with respect to frequency and phase position of its output voltage, a controllable switching device with a load operation switching state, in which the drive motor is coupled to the line voltage connection, and a no-load operation switching state, in which the drive motor is coupled to the frequency changer, so that the drive motor in load operation is fed with a line voltage with essentially constant line frequency and in no-load operation with the output voltage of the frequency changer, a phase difference determination device, by means of which the difference between the phase position of the line frequency and the phase position of the output frequency of the frequency changer can be determined before switching from load operation to no-load operation, and a phase control device, by means of which the phase position of the output frequency of the frequency changer can be controlled as a function of the recorded phase difference in essential agreement with the phase position of the line frequency, switching of the switching device being controllable as a function of achievement of such phase agreement.
In one embodiment of the invention, in conjunction with switching from no-load operation to load operation, a ramp-like rise of the output frequency of the frequency changer is initially produced before the output frequency of the frequency changer is brought to the line frequency and switched from frequency changer feed to line voltage feed. Likewise, during switching from load operation to no-load operation, a ramp-like decline in output frequency of the frequency changer can be produced, after switching from line voltage feed to frequency changer feed has occurred. In this manner, a situation is achieved in which the movement speed of the conveyor device both during the transition from no-load operation to load operation and during the transition from load operation to no-load operation changes gently and therefore free of jolts.
In one embodiment of the invention, switching between no-load operation and load operation occurs by means of a switching device that has a first controllable switching device that connects the drive motor to the line voltage connection and a second controllable switching device that connects the drive motor to the frequency changer, in which only one of the two switching devices is connectable conducting and that switching of the nonconducting switching device to the conducting state is only possible after a predetermined currentless period following switching of the switching device that had been conducting to the nonconducting state. This allows for the fact that the contactors ordinarily used for such switching devices do not operate free of delay and ensures that simultaneous conduction of both switching devices does not occur, which could result in a hazardous short circuit of the line voltage through the frequency changer.
During the currentless period the drive motor remains without power supply, which leads to a drop in speed of the drive motor during the currentless period because of slip of the drive motor and inherent friction of the conveyor device, so that a reduction in the magnitude and frequency of the motor terminal voltage occurs.
In order to avoid adverse effects of smooth switching between no-load operation and load operation by these phenomena connected with the currentless periods, in one embodiment of the invention a voltage determination device is provided, by means of which the motor terminal voltage is determined at least during the currentless period. The output voltage of the frequency changer is brought to the determined motor terminal voltage before switching of the drive motor to frequency changer feed. Transient currents during switching between load operation and no-load operation are therefore minimized.
Determination of the motor terminal voltage can occur by means of a voltage measurement device. Since the motor data and the currentless period are normally known for a specific conveyor device, the drop in motor terminal voltage occurring during the currentless period can also be determined from these data. In this case a motor voltage measurement device is not necessary.
Because of the mentioned measures, a situation is achieved in which, during switching from load operation to no-load operation, i.e., during switching from line voltage feed to frequency changer feed at the time at which the motor is connected to the output of the frequency changer, the output voltage of the frequency changer is adapted in terms of voltage and phase to the motor terminal voltage, the motor speed and the motor rotational position of the drive motor.
Since the speed of the drive motor diminishes during the currentless period, one embodiment of the invention proposes that the frequency changer run the drive motor during switching from no-load operation to load operation before the switching process at a speed that lies above the motor speed corresponding to the line frequency by the amount that the motor speed drops during the currentless period. The amount by which the motor speed drops during the currentless period can be determined for the corresponding conveyor device, for example, by measurement, and allowed for during design of the control of the frequency changer.
Ordinary frequency changers have bridge circuits in their output stage containing electronic switches that are controlled with switch control pulses, whose frequency determines the output frequency of the frequency changer. The already discussed control of the voltage value of the frequency changer output voltage is produced in one embodiment of the invention by pulse width modulation of the switch control pulse.
In one embodiment of the invention, a Schmitt trigger circuit is used to determine the phase difference between the phase position of the line voltage and the phase position of the output voltage of the frequency changer, by means of which the time of passage through predetermined threshold values either on the rising flank or falling flank of the line voltage and frequency changer output voltage, for example, the zero passage, is determined. The phase difference can be determined from the time difference of these times.
In one embodiment of the invention, a counter is used to determine the phase difference between the phase position of the line voltage and the phase position of the output voltage of the frequency changer, said counter counts the number of clock pulses of a clock generator occurring between the two mentioned times. The counter is started at the time at which the Schmitt trigger circuit determines achievement of the predetermined threshold value of the line voltage. The counter is stopped at the time at which the Schmitt trigger circuit then determines achievement of the predetermined threshold value of the output voltage of the frequency changer. From the value of the counter reached at this second time, the phase difference between the line voltage and the frequency changer output voltage is derived. The phase position of the frequency changer output voltage is then corrected as a function of this numerical value in order to bring it into agreement with the phase position of the line voltage before a switch is made from line voltage feed to frequency changer feed.
Schmitt triggers can be used to determine both times, i.e., one to determine the phase position of the line voltage, on the one hand, and one to determine the phase position of the frequency changer output voltage, on the other. Since the phase position of the frequency changer output voltage can be deduced from the pulse-like switch control signals for the switch arrangement controlling the output voltage of an ordinary frequency changer, one can also get by with a single Schmitt trigger. In this case, the phase position of the line voltage is determined with the single Schmitt trigger, the counting process of the counter is started with the output signal of the single Schmitt trigger and stopping of the counter is controlled as a function of the switch control signal for the switch arrangement of the frequency changer that determines the phase position of the frequency changer output voltage.
Especially by the last embodiment, a control device according to the invention can be produced with particularly low demands and at particularly low cost accordingly.
In preferred embodiments of the invention, correction of the phase position of the frequency changer output voltage is carried out as a function of the determined phase difference between the line voltage and the frequency changer output voltage only during switching from load operation to no-load operation, whereas during a switch from no-load operation to load operation starting of the frequency changer output voltage is controlled with an empirically determined rising ramp and with slow adaptation of the phase position of the frequency changer output voltage to the phase position of the line voltage.